This invention relates to data storage and retrieval systems and, more particularly, to a method and apparatus for detecting defective areas on a recording medium and transferring data between the recording medium and a data supply unit, bypassing the defective areas.
Various types of information retrieval systems have been developed including those which utilize bulk storage devices for processing digital data. One type of bulk storage device includes a disk file having at least one magnetic recording disk in which information is stored on the surfaces of the disk. Each surface has a plurality of concentric tracks on which are stored the data, with each track being divided into one or more sectors. Typically, the data is recorded on each track by magnetizing the recording surface. Usually, bits of information are recorded, each of which may be a logic "1" or "0" distinguishable by a change or transition of magnetization of the surface.
In the manufacture of disks, a magnetic coating is formed on the surface of the disk for subsequent magnetization by the disk user in accordance with the information to be stored. However, various problems occur with information storage systems using these magnetic disks due to defective areas on the recording surface. There are areas in which there may be no magnetic coating or an insufficient amount of such coating, i.e., imperfections, to properly record data on the disk, or to properly distinguish magnetic transitions when reading the data. One problem involves data integrity in that the user may not be assured that the data is being properly written on or read from the recording surface due to the defective areas. Another problem is that if the user is reading incorrect data from the recording surface due to the defective areas, then additional time might be used to reread the data until the desired information is retrieved, thereby undesirably increasing throughput.
As a result of these problems, both the manufacturer of the disks, and the manufacturer of disk drives which write and retrieve the data on the disks, have taken precautions to minimize the effects of defective areas on recording surfaces. After the magnetic coating is placed on the recording surface, the disk manufacturer will test the disk before distributing it for use with the disk drives. If too many defective areas are found, the disk will be discarded; otherwise, it will be distributed to the user with a specified number of defective areas on it. This specification may not be entirely satisfactory since during use of the disk additional defective areas may appear if the disk wears or dirt gets imbedded on the disk, or the original defective areas may grow in size.
The disk drive manufacturer, on the other hand, may include in the drive or control unit a circuit to detect the defective areas to record the data on other areas of the recording surface. Consequently, the disk manufacturer will provide spare areas on which to record data which cannot be written on the defective areas. In one solution, the disk will have spare data tracks, while in another approach the disk will have one or more spare sectors for a given track. Upon detection of a sector of a given track having a defective area, the disk drive circuit will cause a relocation of all the data intended for that sector onto the spare track or the spare sector of the given track, depending on the system being used. In retrieving the recorded data, when the sector having the defective area is detected, the data will then be read either from the spare track or from the spare sector, depending on the system being used.
Either of the above disk drive techniques for avoiding recording on the defective areas results in a loss in data storage capacity. The more spare tracks or sectors needed to account for all the defective areas existing or anticipated on the recording surface, the less capacity there is on the disk for storing the data. Also, the time for accessing the data on recording surfaces having spare recording areas may be significantly and undesirably increased. For example, in a disk having the spare tracks, these may be the inner concentric tracks of the recording surface. If data is being read or written on an outer track on the recording surface, and a sector of this outer track having a defective area is detected, then additional time will be required to move the recording head to an inner spare track to read or write the desired data. Additional time may also then be needed to return the recording head to the outer track to continue reading or writing data on this outer track. In a system using a spare sector of a given track, when a sector having a defective area is detected, the data transfer operation may have to cease temporarily until the disk can be rotated to position the spare sector under the recording head; thereafter, the data transfer operation again may have to cease until the disk again rotates to position an appropriate nondefective sector of the track under the recording head for reading or writing the data.
Another disadvantage with the above disk drive techniques relates to small, newly grown defective areas which cannot be detected due to the sensitivity of the disk drive detector circuit. This will result in a loss in data integrity which may require the disk drive user to reread or rewrite data on the recording surface, thereby undesirably increasing throughput. Alternatively, the user of the disk drive would have to take into consideration these undetectable defective areas on a recording surface when planning to use the storage system since such areas may reduce the specified capacity of the disk. In other words, in utilizing such disk drive, the user knows that the disk drive may not be optimum in the sense that additional and undetectable areas will have to be accounted for in planning the amount of data that can be stored on the disk.